rtlwifi: Fix many compile warnings when using W=1

Many warnings like the following arise from a build with W=1 on the
make line:

warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Changing the overall debug level storage from unsigned to signed fixes these.
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rtlwifi/wifi.h

@@ -1702,7 +1702,7 @@ struct rtl_works {
 
 struct rtl_debug {
 	u32 dbgp_type[DBGP_TYPE_MAX];
-	u32 global_debuglevel;
+	int global_debuglevel;
 	u64 global_debugcomponents;
 
 	/* add for proc debug */